Conventional helicopters have reached a performance plateau, at least in terms of forward flight speed capability. Of course, even with such performance limitations, helicopters remain extremely useful flying machines in a wide range of military and civilian applications. However, helicopters (as well as other like rotorcraft) could potentially be even more useful if they could travel faster. The potential advantages in both military and civilian applications of a helicopter (or other similar rotorcraft) which is capable of higher speed forward flight will be readily apparent to those skilled in the art.
A number of developments and investigations in this area have been carried out in previous decades, and some continue today. Some examples of previous efforts to produce helicopters that are capable of higher speed forward flight are discussed briefly below. Note that the previous examples discussed below are mainly compound helicopters (almost gyrocopters). A compound helicopter being considered to be a helicopter that has at least one separate rotor (separate from the main rotor) for producing forward thrust.
As alluded to above, the potential usefulness of an aircraft which is capable of vertical take-off and landing, hover, and reasonably high-speed forward flight is apparent from the numerous previous efforts that have been made in pursuit of this goal. Recent investigations by major OEMs, and the push by various militaries for an increase in forward flight speed capability for helicopters and other rotorcraft, also suggests that there is a need for advancement in this area. However, the ability to provide an aircraft which is capable of vertical take-off and landing, hover, and forward flight at speeds higher than are currently possible should preferably also be achieved at a reasonable cost. That is, at a reasonable cost in terms of the financial/economic costs, and also in terms of costs or detriments in relation to other aspects of aircraft performance.
Two relatively recent attempts at producing an aircraft capable of vertical take-off and landing, hover, and higher-speed forward flight have been made by Sikorsky and Airbus Helicopters, respectively.
Whilst the configurations of the Sikorsky X2 and the Airbus Helicopters X3 differ considerably from each other in many respects, both are able to provide not only forward thrust for increased forward flight speed, but also the anti-torque forces necessary to balance the rotorcraft in hover and low speed flight. However, both configurations involve a new overall rotorcraft design. There is therefore little or no possibility for retrofit or conversion to adapt existing helicopters to these configurations and the likelihood is that the resulting production aircraft will be high in cost.
In a conventional single-main-rotor helicopter as schematically illustrated in FIG. 1, the main rotor provides lift and forward thrust as well as pitch and roll control. Therefore, for a conventional single-main-rotor helicopter to travel forward at speed, the main rotor must be tilted forward to provide a horizontal thrust component. The horizontal component of the total force produced by the main rotor must match the drag of the whole rotorcraft in order to maintain a constant speed. Also at high speed, the drag from air resistance on the aircraft is high. At high speed and high drag, the amount of lift and forward thrust that can be produced by the main rotor disk becomes limited because of the compounding effects of main rotor disk tilt and retreating blade of stall. In fact, retreating blade stall is one of the main factors that limits further increases in forward speed capability for conventional single-main-rotor helicopters.
Providing a separate rotor that directly generates forward thrust (e.g. like in the examples of the Sikorsky X2 and the Airbus X3) relieves the main rotor of the requirement to generate forward thrust. The main rotor is therefore only required to generate lift (although it must still also provide pitch and roll control forces). In any case, where one or more separate rotors are provided for providing forward thrust (meaning that the main rotor is not required to generate forward thrust) the main rotor disk therefore does not need to be tilted. In other words, because the main rotor disk does not need to be tilted, the angle of attack of the retreating blade does not need to be increased quite as much in order to balance the lift forces produced by the respective retreating-blade-side and advancing-blade-side of the disk. Accordingly, retreating blade stall is delayed and the result is that the maximum forward velocity achievable by the single-main-rotor helicopter can be increased.
Experimental compound helicopters have in the past achieved considerably higher speeds than conventional rotorcraft, with top speeds in the order of 400-500 km/hr. This suggests that compound helicopters may be able to provide a speed advantage of around 30-50% or more over conventional helicopters. Compound helicopters have also been found to benefit from improved specific fuel consumption resulting in an increase in range compared with conventional helicopters.
In view of the foregoing, in the context of helicopters, it would appear to be desirable if a propulsion system for a helicopter could be provided which is capable of producing the lateral anti-torque forces required to stabilise and manoeuvre a helicopter in hover, and which is also capable of producing, when required, a direct forward thrust force for forward flight so as to help achieve higher forward flight speed, and preferably with greater efficiency, than is currently possible. In order to reduce costs and possibly also allow for retrofit or conversion of current helicopters, it may also be preferable to avoid an entirely new overall aircraft design, and to instead provide a comparatively simpler tail-rotor system.
It is to be clearly understood that mere reference herein to previous or existing apparatus, systems, methods, practices, publications or other information, or to any problems or issues, does not constitute an acknowledgement or admission that any of those things individually or in any combination formed part of the common general knowledge of those skilled in the field, or that they are admissible prior art.